Preparation of halophosphate phosphors using digestion formed apatite

ABSTRACT

Apatite-type alkaline-earth metal halophosphate phosphors are prepared by a double digestion process. Unactivated alkalineearth metal fluoro-apatite is formed by digesting an alkalineearth metal acid phosphate and a fluoride of the same alkalineearth metal. The unactivated fluoro-apatite is then digested in a solution containing activators to form activated alkaline-earth metal fluoro-apatite which is further processed to provide the phosphor. An improved apatite crystal structure is obtained by forming the apatite structure before the activators are added and the digesting of acid phosphates and fluorides provide a fast and inexpensive method of forming such an apatite. Preferably, the alkaline-earth metal is calcium and chlorine is added during firing so that a calcium fluoro-chloro-phosphate phosphor is formed.

United States Patent 1191 y 1 1 PREPARATION OF HALOPHOSPHATE PHOSPHORSUSING DIGESTION FORMED APATITE [75] inventor: Rudolph Nagy, Lac duFlambeau,

21 Appl. No.: 205,128

52 us. Cl. 252/3014 P 51 Int. Cl C09k H36 [58] Field of Search 252/3014P, 301.6 P

[56] References Cited UNlTED STATES PATENTS a 3,538,014 11/1970 Wachtel252/3014 P 3,575,876 4/1971 Piper 252/3014 P FOREIGN PATENTS QRAPPLICATIONS Great Britain 252/30l.4 P

Primary Examiner-Oscar R. Vertiz Assistant Examiner-J. CooperAttorney-A. T. Stratton et al.

Apatite-type alkaline-earth metal halophosphate phosphors are preparedby avdouble digestion. process. Unactivated alkaline-earth metalfluoro-apatite is formed by digesting an alkaline-earth metal acidphosphate and a fluoride of the same alkaline-earth metal. Theunactivated fluoro-apatite is then digested in a solution containingactivators to form activated alkaline-earth metal fluoro-apatite whichis further processed to provide the phosphor. An improved apatitecrystal structure is obtained by forming the apatite structure beforethe activators are added and the digesting of acid phosphates andfluorides provide a fast and inexpensive method of forming such anapatite. Preferably, the alkaline-earth metal is calcium and chlorine isadded during firing so that a calcium fluoro-chloro-phosphate phosphoris formed.

7 Claims, 1 Drawing Figure PAIENTEIHugzams 3 3.755187,

DIGESTING C0HPO4 WITH CuFg TO FORM UNACTIVATED CALCIUM FLUORO-APATITEDIGESTING THE UNACTIVATED CALCIUM FLUORO-APATITE WITH A Sb ORA Mn-SbSOLUTION TO FORM ACTIVATED CALCIUM FLUORO-APATITE WAS HIN G TH E FLUORO-APATI TE FIRING THE FLUORO-APATITE IN A CHLORINE CONTAINING ATMOSPHERETO FORM A CALCIUM FLUORO-CHLORO-APATITE PHOSPHOR PREPARATION OFHALOPHOSPI-IATE PHOSPHORS USING DIGESTION FORMED APATITE ICROSS-REFERENCE TO RELATED APPLICATIONS In copending applications Ser.No. 77,701, filed Oct. 2, 1970 and on which Ser. No. 266,792 was filedas a continuation on June 27, 1972; Ser. No. 71,204, filed Sept. 10,1970; and Ser. No. 109,988, filed Jan. 26, 1971 and on which U.S. Pat.No. 3,694,371 issued on Sept. 26, 1972, and owned by the presentassignee are disclosed several methods for precipitating activatedapatites and processing them into halophosphate phosphors. ln copendingapplication Ser. No. 87,578, filed Nov. 6, 1970 on which US. Pat. No.3,663,473 issued on May 16, 1972, and owned by the present assigneee, isdisclosed a method in which an apatite is precipitated which containsone activator, a second activator is added by digesting after the pH isadjusted, and the apatite is then further processed to form ahalophosphate phosphor.

BACKGROUND OF THE INVENTION This invention relates to a method ofproducing halophosphate phosphors. The halophosphate phosphor familyremains the most important commercially useful group of phosphors. Themanganeseand antimonyactivated calcium fluoro-chloro-apatite with orwithout supplementation by cadmium is the standard fluorescent lampphosphor presently used. This phosphor is typically prepared by a solidstate firing of the raw mix constituents, wherein the apatite structuredphosphor is formed at a very high firing temperature. The crystalstructure formed by such solid state reactions is hampered by transportand mobility limitations of the solid state reaction. The firing of rawmix constituents also requires costly furnace equipment and requiresconsiderable time for firing, cooling, and grinding the material to afinely divided status.

In the aforementioned copending applications and in US. Pat. No.3,538,014 issued Nov. 3, 1970 and U.S. Pat. No. 3,575,876 issued April20, 1971, methods are disclosed for circumventing the difficulties ofsolid state firing, which methods form an apatite by precipitation.These precipitated apatites are then further processed to formhalophosphate phosphors. For commercial production relatively largevessels are required for precipitation techniques as the volumes of theliquids used are large compared to the volume of the precipitatedapatite.

SUMMARY OF THE INVENTION A method is disclosed whereby apatiteis formedby digesting (sometimes referred to as slurrying), rather than formingthe apatite by solid state firing or by precipitation. As unactivatedalkaline-earth metal fluoroapatite is formed by digesting predeterminedamounts of an alkaline-earth metal acid phosphate and a fluoride of thesame alkaline-earth metal which amounts are included in aproximatelystoichiometric quantities as required to form the apatite. Theunactivated fluoro-apatite is digested in a solution containingpreselected activators to form an activated alkalineearth metalfluoro-apatite. The activated fluoro-apatite is then washed and fired,under predetermined conditions to optimize the luminescence. Chlorinemay be added during the firing process such that of fluorochloroapatitephosphor is formed.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of theinvention reference may be had tothe accompanying drawing. The soleFIGURE is a flowchart illustrating the steps of a preferred method ofpreparing halophosphate phosphors in which the apatite is-formed by adigestion process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I With reference to the soleFIGURE there is shown a flowchart of a preferred method used to producehalophosphate phosphor in accordance with this invention. Unactivat'edcalcium fluoro-apatite is formed by a novel digestion of calcium acidphosphate with calcium fluoride. The reaction 9CaHPO CAF 3Ca,.(PO -CaF3H -,PO is unexpected because Cal is very insoluable in water. Thisdigestion process between two particulate compounds is relatively rapid,generally requiring only 0.5 to 5 hours, and can take place in arelatively small vessel as both of the reaction constituents are inparticulate form. The digestion may be continued until a predeterminedpH IS achieved or for a predetermined period of time.

The unactivated calcium fluoro-apatite is then digested with a solutionwhich contains antimony or antimony and manganese ions, thereby formingactivated calcium fluoro-apatite. While it would be possible toincorporate the activators in the same digestion process in which theapatite was formed, it has been found that a better apatite structure isformed when the activators are added after the apatite is formed.

The fluoro-apatite is then washed. The wash may be initially with hotwater rinses which are followed by an alcohol rinse.

The fluoro-apatite is then fired at above 1000C and preferably at about1 180C for one to three hours. The atmosphere preferably containschlorine such that a fluorochloro-apatite phosphor is formed.

The following examples illustrate this invention:

EXAMPLE 1 The following ingredients are heated to 40-100C and stirred:132 grams of CaHPO 7.8 grams of CaF,, 7.0 grams of NH,F, and 2000milliliters of water. When the pH of the above mixture has decreased to2.5 T0 3.5 (approximately one hour) the following activator solution isadded: 400 milliliters of water (at approximately 100C), 3.8 grams ofMn(C,I-I,O,),'4H O, 1.6 grams'of Cd(C,l-l O,),-3H,O, and 0.8 gram ofSbCl The mixture is further stirred for approximately one hour, washedtwo times by decantation and then washed with hot water on a filter. Theprecipitate is I then fired in a nitrogen atmopshere at about 1000C forapproximately three hours to form a calcium fluoroapatite phosphor.

EXAMPLE 2 The digestion of calcium acid phosphate and calcium fluorideas described in Example 1 can be further speeded by the addition ofcitric acid in the amount of 0.1 to 5 percent of the water during thefirst digestion. This provides the added advantage that new calciumphosphates other than apatite will probably not be formed, as apparentlyall calcium phosphates other than apatite are soluble in citric acid.

While the above examples have been given using calcium as thealkaline-earth metal, the procedures also are effective for otheralkaline-earth metals such as strontium.

While antimony or manganese and antimony are used as activators in theforegoing description, other activators, such as divalent europium, canbe substituted.

I claim as my invention:

1. A method of preparing an activated alkaline-earth metal halophosphatephosphor having an apatite crystal structure, which method comprises:

a. forming an unactivated alkaline-earth metal fluoro-apatite byslurrying in an aqueous medium predetermined amounts of a particulatealkaline-earth metal acid phosphate and a particulate fluoride of thesame alkaline-earth metal, and said predetermined amounts being includedin said aqueous medium in approximately stoichiometric quantities asrequired to form said apatite;

b. slurrying said formed unactivated fluoro-apatite in an aqueous mediumcontaining in solution preselected activators for said phosphor, to formactivated alkaline-earth metal fluoro-apatite; c. washing said activatedfluoro-apatite; and d. firing said washed fluoro-apatite at above l000Cfor a time sufficient to optimize the luminescence. 2. The method asspecified in claim 1, wherein chlorine is incorporated into saidhalophosphate phosphor by admixing a readily volatilizablechlorine-containing compound with said washed fluoro-apatite prior tofiring or by firing said washed fluoro-apatite in a chlorinecontainingatmosphere.

3. The method as specified in claim 1, wherein said activators containedin said solution consist essentially of antimony or manganese andantimony.

4. The method as specified in claim 1, wherein said alkaline-earth metalis calcium and said unactivated fluoro-apatite is formed by slurryingCal-lPO. and CaF, in water.

5. The method as specified in claim 4, wherein citric acid in the amountof 0.1 to 5.0 percent of said water is included during said slurrying ofsaid CaHPO and CaF,.

6. The method as specified in claim 1, wherein said acid phosphate andsaid fluoride are slurried at 40lOOC for a predetermined period of timeof at least 0.5 hour.

7. The method as specified in claim 1, wherein said acid phosphate andsaid fluoride are slurried until a pH of 2.5 to 3.5 is achieved.

2. The method as specified in claim 1, wherein chlorine is incorporatedinto said halophosphate phosphor by admixing a readily volatilizablechlorine-containing compound with said washed fluoro-apatite prior tofiring or by firing said washed fluoro-apatite in a chlorine-containingatmosphere.
 3. The method as specified in claim 1, wherein saidactivators contained in said solution consist essentially of antimony ormanganese and antimony.
 4. The method as specified in claim 1, whereinsaid alkaline-earth metal is calcium and said unactivated fluoro-apatiteis formed by slurrying CaHPO4 and CaF2 in water.
 5. The method asspecified in claim 4, wherein citric acid in the amount of 0.1 to 5.0percent of said water is included during said slurrying of said CaHPO4and CaF2.
 6. The method as specified in claim 1, wherein said acidphosphate and said fluoride are slurried at 40*-100*C for apredetermined period of time of at least 0.5 hour.
 7. The method asspecified in claim 1, wherein said acid phosphate and said fluoride areslurried until a pH of 2.5 to 3.5 is achieved.